niece in bad situation

What is the name of your state? nevada
It was recently brought to my attention that my niece's mother(my ex sister-in-law) has become involved in prostitution. Obviously this sickens me, my niece just turned 5 and she has a little sister just 18 months (not my niece,different dad) From what I understand the mother is not involved in "legitimate prostitution" (that would be just as disgusting but legal) but for lack of a better term, free-lances. My niece has told me that she and her sister have been left alone at night,they live in Reno,so I find this alarming.
I live a couple hundred miles from Reno as does my brother the dad,so we're not able to be there to do anything about this.
I don't approve of prostitution,but at the same time I don't care what my x sister-in-law does to herself,it's the situations that my niece has been and will be put in that concern me.There have already been numerous domestic violence calls to the police,and my poor little peanut has seen her mom's boyfriend beat her grandpa bloody,threaten her mother and another boyfriend removed my niece from their apartment because her mom was breaking up with him, the mom has had 3 years (since her divorce from my brother)to get it together and continues to find herself and her kids in the same situations. It's very sad and nervracking! we are at present working on verifying all the information we have received,and are planning on spending this week watching the apartment for any times she leaves the girls alone. A private investigator may be needed anyway,but we hope to be able to catch her on our own. When we have proof she's leaving these kids alone what can I do (the aunt) to get temporary guardianship of my niece? Or custody, if the mom doesn't want to make any changes? I love my brother dearly but by his own admission,he does not want custody or to be a fulltime parent.
This makes me sad,but it doesn't change my feelings for my niece.
Any suggestions and information would be greatly appreciated.

"but we hope to be able to catch her on our own."
*** And if you do, your 'proof' will not be accepted. Simply, you and your 'associate' are tainted by your involvement. If you truly want to try to help, get a removed third party (investigator).

"When we have proof she's leaving these kids alone what can I do (the aunt) to get temporary guardianship of my niece?"
*** Pretty much nothing. If you can prove that this child is in an unsafe environment, contact the state CPS (or equal) and file a complaint. They will investigate and if warranted, remove the child(ren) from the home.

"Or custody, if the mom doesn't want to make any changes?"
*** If CPS does take custody, they will place them in whatever home (yours or someone elses) that is best for them.

If you are POSITIVE that these kids are alone, then call the police and/or CPS immediatley. Also tell your brother to be a man....step up and take care of his child. Does he not care about the environment that his kid is living in? Where is the father of the other child? Do you know who he is, and if he would care what's going on with his child? I would get a PI and have everything documented.